Imaging system

ABSTRACT

Toner image degradation due to excessive buildup of thick deposits of a solid stable hydrophobic metal salt of a fatty acid on the surface of electrostatographic imaging members during successive imaging operations with development material containing the metal salt is prevented by continuously or intermittently maintaining the thickness of the buildup below about 10 microns by vigorous rubbing contact with a wiping member.

United States Patent Palermiti et a1.

1151 3,635,704 1 51 Jan. 18,1972

HMAGING SYSTEM Inventors: Frank M. Palermiti, 5 Framingham Lane, Pittsford, NY. 14534; Stephen F. Royka, 265 Howell Road, Fairport, NY. 14450; Wllliam A. Sullivan, Jr., 638 Harriswood Drive, Webster, NY. 14581; Robert L. Emerald, 182 C Brambury Drive, Rochester, NY. 14621 Filed: Feb. 1, 1968 App1.No.: 702,194

Related US. Application Data Continuation-impart of Ser. No. 511,242, Dec. 2, 1965, abandoned.

us. c1 ..96/1, 96/14, 117/175,

252/621 ..G03g 9/00, 003 13/00 ..96/1; 252/62.l; 355/15 References Cited UNITED STATES PATENTS Beyer ..117/37 LX X Primary ExaminerGe0rge F. Lesmes Assistant Examiner-John R. Miller Attorney-James J. Ralabate, Norman E. Schrader and Ronald Zibelli [5 7] ABSTRACT Toner image degradation due to excessive buildup of thick deposits of a solid stable hydrophobic metal salt of a fatty acid on the surface of electrostatographic imaging members during successive imaging operations with development material containing the metal salt is prevented by continuously or intermittently maintaining the thickness of the buildup below about 10 microns by vigorous rubbing contact with a wiping member.

13 Claims, No Drawings llVlAGllNG SYSTEM This application is a continuation-in-part of copending application Ser. No. 511,242, filed Dec. 2, 1965, now abandoned. This invention relates to imaging systems, and more particularly, to an improved development process.

BACKGROUND OF THE INVENTION The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic xerographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placinga uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting latent electrostatic image by depositing on the image a finely divided electroscopic material referred to in the art as toner. The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the latent electrostatic image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by unifonnly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.

Several methods are known for applying the electroscopic particles to the latent electrostatic image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552, is known as cascade" development. In this method, a developer material comprising relatively large carrier particles having fine toner particles electrostatically coated thereon is conveyed to and rolled or cascaded across the electrostatic image-bearing surface. The composition of the carrier particles is so chosen as to triboelectrically charge the toner particles to the desired polarity. As the mixture cascades or rolls across the image-bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier and excess toner are them recycled. This technique is extremely good for the development of line copy images.

Another method of developing electrostatic images is the magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method, a developer material containing toner and magnetic carrier particles is carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carriers into a brushlike configuration. This magnetic brush" is engaged with the electrostatic image-bearing surface and the toner particles are drawn from the brush to the latent image by electrostatic attraction.

Still another technique for developing electrostatic latent images is the "powder cloud process as disclosed by C. F. Carlson in U.S. Pat. No. 2,221,776. In this method, a developer material comprising electrically charged toner particles in a gaseous fluid is passed adjacent the surface bearing the latent electrostatic image. The toner particles are drawn by electrostatic attraction from the gas to the latent image. This process is particularly useful in continuous tone development.

Any other development method such as touchdown development, as disclosed by R. W. Gundlach in U.S. Pat. No. 3,166,432 may be used where suitable.

In automatic xerographic equipment, it is conventional to employ a xerographic plate in the form of a cylindrical drum which is continuously rotated through a cycle of sequential operations including charging, exposure, developing, transfer and cleaning. The plate is usually charged with corona of positive polarity by means of a corona generating device of the type disclosed by L. E. Walkup in U.S. Pat. No. 2,777,957 which is connected to a suitable source of high potential. After forming a powder image on the electrostatic latent image during the development step, the powder image is electrostatically transferred to a support surface by means of a coronagenerating device such as the corona device mentioned above. In automatic equipment employing a rotating drum, a support surface to which a powder image is to be transferred is moved through the equipment at the same rate as the periphery of the drum and contacts the drum at the transfer position interposed between the drum surface and the corona generating device. Transfer is effected by a corona generating device which imparts an electrostatic charge to attract the powder image from the drum to the support surface. The polarity of charge required to effect image transfer is dependent upon the visual form of the original copy relative to the reproduction and the electroscopic characteristics of the developing material employed to eflect development. For example, where a positive reproduction is to be made on the positive original, it is conventional to employ a positive polarity corona to effect transfer of a negatively charged toner image to its support surface. When a positive reproduction from a negative original is desired, it is conventional to employ a positively charged developing material which is repelled by the charged areas on the plate to the discharged area thereon to form a positive image which may be transferred by negative polarity corona. In either case, a residual powder image usually remains on the plate after transfer. Before the plate may be reused for a subsequent cycle, it is necessary that the residual image by removed to prevent ghost images" from forming on subsequent copies. In the positive-to-positive' reproduction process described above, the residual developer powder is tightly retained'on the plate surface by a phenomenon that is not fully understood but believed caused by an electrical charge that prevents complete transfer of the powder to the support surface, particularly in the image area. This charge is substantially neutralized by means of a corona device prior to contact of the residual powder image with a cleaning device. The neutralization of the charge enhances the cleaning efficiency of the cleaning device.

Various electrostatographic plate cleaning devices such as the brush cleaning apparatus and the web cleaning apparatus are known in the prior art. A typical brush cleaning apparatus is disclosed by L. E. Walkup et al. in U.S. Pat. No. 2,832,977. The brush type cleaning means usually comprises one or more rotating brushes which brush residual powder from the pate into a stream of air which is exhausted through a filtering system. A typical web cleaning device is disclosed by W. P. Graff, Jr. et al. in U.S. Pat. No. 3,'l86,838. As disclosed by Graff, Jr. et al., removal of the residual powder on the plate is effected by passing a web of fibrous material over the plate surface.

While ordinarily capable of producing good quality images, conventional developing materials suffer serious deficiencies in certain areas. The developing materials must flow freely to facilitate accurate metering and even distribution during the development and developer recycling phases of the xerographic process. Some developer materials, though possessing some desirable properties such as proper triboelectric characteristics, are unsuitable because they tend to cake, bridge, and agglomerate during handling and storage. The triboelectric and flow characteristics of many toners and carriers are adversely affected when relative humidity is high. Some developers contain components which tend to buildup on electrostatographic imaging surfaces thereby adversely affecting the quality of deposited images. This buildup problem is particularly acute in high-speed copying and duplicating machines where contact between the developer and imaging surface occurs a great many more times and at higher velocities then in conventional electrostatographic systems. Numerous known carriers and toners are abrasive in nature. Abrasive contact between toner particles, carriers, electrophotographic surfaces and cleaning devices accelerates mutual deterioration of these components. Replacement of carriers and electrostatic imaging surfaces is expensive and time consuming. Brush cleaning devices must normally be positioned in close proximity to, but out of contact with electrostatographic imaging surfaces to prevent destructive abrasive contact and yet allow the air current generated by the moving brushes to remove residual toner particles. Pressure contact between cleaning webs and imaging surfaces must be kept to a minimum to prevent'rapid destruction of the imaging surface. Friction resulting from high web pressures are occasionally sufficient to cause the drum drive motor to fail due to overheating. Fines formed from toner disintegration tend to drift and form unwanted deposits on critical machine parts. Xerographic copies should possess good line image contrast as well as acceptable solid area coverage. However, when a process is designed to improve either line image contrast or solid area coverage, reduced quality of the other can be expected. Attempts to increase image density by depositing greater quantities of toner particles on the latent electrostatic image are usually rewarded with an undesirable increase in background deposits. Thus, there is a continuing need for a better system for developing latent electrostatic images.

SUMMARY OF THE INVENTlON It is, therefore, an object of this invention to provide a developing system which overcomes the above-noted deficiencies.

It is another object of this invention to provide a process employing developing materials which flow freely.

it is another object of this invention to provide a process employing developing material which possess stable triboelectric properties.

It is another object of this invention to provide a process in which toner particles are readily removable by carriers from backgroundareas during an image development step.

It is another object of this invention to provide a process which prevents undesirable buildup of developer components on reusable elecrostatographic imaging surfaces.

It is another object of this invention to provide a process employing developing materials which reduce mechanical abrasion of reusable electrostatographic imaging surfaces.

It is another object of this invention to provide a process employing developing materials which are effective at low initial electrostatic surface potentials.

It is another object of this invention to provide a process employing developer mixtures which form dense toner images.

It is another object of this invention to provide a process employing developing materials which are resistant to physical degradation.

It is another object of this invention to provide a process employing developer mixtures which are readily transferable from an electrophotographic surface to a transfer surface.

It is another object of this invention to provide a process employing carriers, toners and mixtures thereof having physical and chemical properties superior to those of known developing materials.

It is another object of this invention to provide an electrostatographic process which is superior to known development processes.

The above objects and others are accomplished, generally speaking, by providing an electrostatographic imaging process which includes the steps of developing an electrostatic latent image on the surface of a reusable electrostatographic imaging member with a developer containing a solid stable hydrophobic metal salt of a fatty acid and continuously or intermittently maintaining the thickness of residual material containing the metal salt which deposits on the surface of the imaging member below about 10 microns by bringing a wiping member into vigorous rubbing contact with the deposited residual material. The solid stable hydrophobic metal salt of a fatty acid is present in the developer in an amount from about 0.02 to about 20 percent, based on the weight of the toner, at the surface of the particles in the developing material.

The salt may be compounded into the developing material in any suitable manner to form either a physical mix of salt particles with developing material particles, carriers or toners coated with the metal salt, carriers and/or toners containing the salt as an overall component or combinations thereof. Where the solid hydrophobic metal salt of a higher fatty acid is to be physically mixed with or applied as a coating on toner or carrier particles, the metal salt is preferably present in an amount from about 0.02 percent to about 10 percent based on the weight of the toner in the final developer mixture. Optimum results are obtained with about 0.05 to about 4 percent of the metal salt. Although the initial electrostatic imaging surface potential may be reduced and abrasion resistance improved when the proportion of metal salt present is increased above about 10 percent undesirable background deposits increase noticeably. If the charge voltage is reduced to compensate for the presence of metal salt in excess of about 10 percent, the images begin to acquire a washed out" appearance. It is not essential that the entire surface of each toner particle be coated with the metal salt, e.g., sufficient'metal salt is present when 10 to 16 percent of the toner particle surfaces are coated with a metal salt. When the metal salt is dispersed in rather than coated on a toner or carrier particle, proportionally more metal salt is necessary in order to maintain a sufficient quantity of exposed salt at the surface of the toner or carrier particle. The additional amount of metal salt necessary depends to a large extent on the surface area of the particles, hence upon the particle diameter selected. The degree of rubbing or wiping contact between the electrostatographic imaging surface and the cleaning means employed in the process of this invention should be sufficient to maintain the buildup thickness of the stable solid hydrophobic metal salt of a fatty acid below about 10 microns. The maintenance of buildup thickness below about 6 microns is preferred because toner image density and quality is markedly improved. Optimum results are achieved by maintaining the buildup thickness belowabout 3 microns. When the buildup is maintained below about 3 microns, the toner images are extremely dense and the copy background is substantially free of un' desirable toner deposits. lf desired, the vigorous wiping step may be employed prior to or subsequent to a suitable cleaning step such as the conventional brush," web and cascade" steps known in the art. Surprisingly, severe rubbing or wiping contact may be employed in the process of this invention without encountering the rapid destruction of conventional photoreceptor surfaces and the overtaxing of drum drive motor capacities.

The use of small quantities of calcium stearate as a pigment wetting agent in zinc oxide developing powders is known as disclosed by Greig in US. Pat. No. 3,053,688 at column 5, line 41 and Greig et al. in Canadian Pat. No. 633,458 at column 9 line 8. However, the quantity of calcium stearate used by Greig and Gteig et al. to facilitate the wetting of pigments dispersed in zinc oxide developing powders is insufficient to provide an effective quantity of exposed calcium stearate at the surface of the toner particle for the purposes of the instant invention. When less than about 0.02 percent metal salt based on the weight of the toner is actually available at the surface of the toner particle, its triboelectric, flow, abrasion, transfer and image forming properties are substantially the same as a toner or carrier which does not contain a metal salt of a fatty acid. Obviously, with a given quantity of metal salt based on the weight of the toner, a greater volume of the salt is available at the surface of the toner or carrier when the metal salt is added to a mixture of preformed colored toner particles or carriers when it is intimately dispersed within each toner particle or carrier. lf the concentration of metal salt is increased to the point where the toner consists essentially of 100 percent metal salt, the metal salt will form thick slippery films on the electrostatic image-bearing surface and carrier particles which interfere with proper powder image transfer, background removal and cleaning. U.S. Pat. No. 3,083,117 discloses a method of applying reactive toners containing 100 percent iron stearate to an electrostatic image and then transferring the developed image to a transfer sheet wet with an alcoholic solution of gallic acid. The iron stearate reacts with the gallic acid to form a black reaction product. In addition to the problems encountered when toner containing 100 percent metal salt is employed, electrostatic development methods of the foregoing type require liquid pretreatment of the receiving sheet with an attendant increase in cost and inconvenience. Further, curling, image bleeding, and offset, often occur when moistened receiving sheets are used. Additional equipment to dispose of toxic and inflammable fumes may also be necessary.

Excellent results have been obtained with developers containing zinc stearate. However, any suitable stable solid hydrophobic metal salt of a fatty acid having a melting point greater than about 57 C. may be substituted for Zinc stearate. The metal salt should be substantially insoluble in water. Water soluble metal salts lack the proper electrical properties and are adversely affected by humidity changes normally occuring in the ambient atmosphere. However, a large proportion of salts commonly regarded as insoluble, actually dissolve to a slight extent. To effectively carry out the purposes of this invention, the solubility of the salt should be negligible. The salts having the desired specific characteristics include many salts of saturated fatty acids, unsaturated fatty acids, partially hydrogenated fatty acids and substituted fatty acids and mixtures thereof.

Typical fatty acids from which stable solid hydrophobic metal salts may be derived include: caproic acid, enanthylic acid, caprylic acid, pelargonic acid, capric acid, undecylic acid, lauric acid, tridecoic acid, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, nondecylic acid, arachidic acid, behenic acid, stillingic acid, palmitoleic acid, oleic acid, ricinoleic acid, petroselinic acid, vaccenic acid, linoleic acid, linolehic acid, eleostearic acid, licanic acid, pan'naric acid, gadoleic acid, arachidonic acid, cetoleic acid and mixtures thereof. Typical stable solid metal salts of fatty acids included: cadmium stearate, barium stearate, lead stearate, iron stearate, nickel stearate, cobalt stearate, copper stearate, strontium stearate, calcium stearate, cadmium stearate, magnesium stearate, zinc oleate, manganese oleate iron oleate, cobalt oleate, copper oleate, lead oleate, magnesium oleate, zinc palmitate, cobalt palmitate, copper palmitate, magnesium palmitate, aluminum palmitate, calcium palmitate, lead caprylate, lead caproate, zinc linoleate, cobalt linoleate, calcium linoleate, zinc ricinoleate, cadmium ricinoleate and mixtures thereof.

Any suitable pigmented or dyed electroscopic toner material may be treated with the metal salt of this invention. Typical toner materials include: gum copal, gum sandarac, rosin, cumaroneindene resin, asphaltum, gilsonite, phenolformaldehyde resins, rosin-modified phenolformaldehyde resins, methacrylic resins, polystyrene resins, polypropylene resins, epoxy resins, polyethylene resins and mixtures thereof. Any suitable coating method may be employed to add the metal salt to the developer materials. When the metal salt is applied as a loose powder, it is most effective in the form of finely divided particles. For example, a particle size distribution from about 0.5 microns to about 50 microns produces excellent results. Although improved results are obtained with toner mixtures manufactured by milling the metal salt powder against the surface of a toner particle or dispersing the metal salt throughout the toner particle, it has been found that unexpectedly better results are obtained when the metal salt powder is rolled with the preformed colored toner particles. Although it is not entirely clear, immovable attachment between a portion of the metal salt and toner particles apparently occurs in milled mixtures as a result of friction and possibly fusion between the metal salt. and toner particles whereas a more loose and movable relationship between toner and metal salt particles exists in rolled mixtures. Although it is not entirely clear, immovable attachment occurs between a portion of the metal salt and toner particles in milled mixtures whereas a more loose and movable relationship between toner and metal salt particles exists in rolled mixtures. Xerographic copies developed with the treated mixtures contain substantially fewer toner particles in the background areas than c0- pies made from untreated mixtures. The treated mixtures allow the use of lower initial electrophotographic surface potentials. The treated mixtures also contribute to reduced electrophotographic drum wear, particularly where cleaning is accomplished by a web cleaner such as the web disclosed by W. P. Graff, Jr. et al. in U.S. Pat. No. 3,l86,838. Further, when a web cleaner is employed in systems utilizing the treated developer, the web pressure and drum speed can be increased without adversely affecting drum or drum drive motor life. The unexpectedly better results obtained from employment of toner mixtures containing metal salt particles may be due to many factors. For example, it is postulated that the increased hydrophobic surface area presented by the metal salt powder may provide a low humidity atmosphere for the toner particles; the slippery metal salt appears to reduce friction during the development and cleaning processes; and the metal salt may reduce Van der Waal forces between the toner particles and the carrier surfaces. in a preferred embodiment, the toner mixture contains colored preformed resin particles comprising 60 to percent styrene or styrene homologues such as those disclosed by C. F. Carlson in U.S. Pat. No. 25, 136 and M. A. lnsalaco in U.S. Pat. No. 3,079,342 mixed with about 0.01 percent to about 4 percent finely divided zinc stearate powder such as Aero 4S manufactured by American Cyanamide. These stable free'flowing developer mixtures produce line and solid area images of especially high density at low initial charging voltages. Drum wear and background deposits are markedly reduced. In addition to rolled and milled mixtures and combinations thereof, the metal salt may be intimately dispersed in each toner particle. However, this embodiment is less desirable than the rolled or milled mixtures because a greater quantity of metal salt is required to provide a sufficient quantity of metal salt, exposed at the surface of the toner particles. When the toner mixtures are intended for use in cascade processes, the toner should have an average particle diameter less than about 30 microns and preferably between 2 and about 15 microns for optimum results. For use in powder cloud development methods, particle diameters of slightly less than 1 micron are preferred.

Suitable coated and uncoated carrier materials are well known in the art. Typical carriers include: sodium chloride, ammonium chloride, aluminum potassium chloride, Rochelle salt, sodium nitrate, aluminum nitrate, potassium chlorate, granular zircon, granular silicon, methylmethacrylate, glass and silicon dioxide. The carriers may be used with or without a coating. Many of the foregoing and other typical carriers are described by L. E. Walkup et al. in U.S. Pat, No. 2,638,416 and E. N. Wise in U.S. Pat. No. 2,618,552. The stable solid hydrophobic metal salts of this invention may be dispersed throughout each carrier particle or coated onto preformed carrier particles. The metal salt may be applied to preformed carriers in any suitable form such as a loose powder, a melt, a solution. emulsion or as a component in a filth-forming binder composition. The loose powder may be sprinkled onto or ground against the surface of the carrier particles. Liquids containing the metal salt may be applied to carriers by any conventional method, e.g., spraying or dip coating. Any suitable film-forming binder having proper triboelectric properties may be used with the metal salts of this invention. When filmforming material of the proper density and surface tension is selected, the metallic salt particles rise to the externally exposed surface of the coating and form a concentrated layer of metal salt which allows the use of lower charging voltages and markedly reduces drum wear and carrier degradation. Typical film-forming binders include compositions comprising vinyl chloride-vinyl acetate copolymers, phenolformaldehyde resins, vinylidene chloride resins, butadiene polymers, melamine-type resins, nitrocellulose, ethylcellulose, parasulfonamide resins, alkyd resins, silicone resins, acrylic ester resins and mixtures thereof. A preferred film-former having the proper density and viscosity is ethylcellulose. it is preferred because it produces superior results. The quantity of metal salt necessary to provide the improved results of this invention is determined by the manner in which the metal salt is incorporated into the carrier material and the quantity of toner material employed in the final developer mixture. Generally speaking, satisfactory results are obtained when about 1 part toner is used with about to 200 parts carrier. Since about 0.02 to about 20 percent, based on the weight of the toner, of metal salt is effective when available at the surface of the developer materials, about 0.0001 to about 2 percent metal salt, based on the weight of the carrier, is desirable at the surface of the carrier particles when the carrier particles, rather than the toner particles are treated with the salt.

The buildup of stable solid hydrophobic metal salts on the surface of a photoreceptor may be regulated by means of anysuitable regulating device. In a preferred embodiment, the regulating device comprises a wiping blade. Wiping blades are preferred because they are simple in construction, inexpensive, do not require frequent replacement, do not normally require a separate power source for operation and occupy a relatively small space in automatic copying or duplicating machines. The principal criteria for selecting a specific wiper blade configuration is that the wiper blade or blades be capable of maintaining the buildup thickness of the stable solid hydrophobic metal salt of a fatty acid below about 10 microns. The wiper blade may be flexible or substantially rigid. Typical rigid wiper blade material include relatively inflexible organic or inorganic materials such as aluminum and copper. Typical flexible blade materials include: polyurethanes, Teflon, polypropylene, natural rubber, polysiloxane rubber and cork. The amount of blade pressure against the surface of a photoconductor sufficient to regulate metal salt buildup depends upon factors such as the relative speed of the blade and the drum, the number of blades, the particular blade material selected, and the concentration of metal salt in the developer. For example, satisfactory results are obtained when a blade pressure of about 9 pounds per square inch is employed in a system operated at a relative rubber blade photoreceptor surface speed of about 3 inches per second and with a metal salt concentration in the developer of about one percent by weight based on the total weight of the toner. L

The buildup of stable solid hydrophobic metal salts on the reusable surface of a photoreceptor may also be regulated by means of a fibrous regulating member. Where the regulating member is a fibrous web, the web may be a woven or a nonwoven material. Any suitable fibrous material may be employed in the web. Typical fiber materials include natural fibers such as cotton, wool, hair and the like and synthetic fibers such as nylon, cellulose derivatives and the like. The amount of web pressure against the surface of the photoconductor sufficient to regulate metal salt buildup depends upon various factors such as the relative speed of the web and the surface of the photoconductor, the surface characteristics of the web, andthe contact time between the web and the photoreceptor surface. Fore example, the metal salt buildup may be satisfactorily controlled by employing a contact pressure of about 10 pounds per square inch when a fresh nonwoven web comprising rayon fibers is moved across the surface of an electrostatographic drum at the relative speed of 3 inches per second for a distance of about inches.

When a brush is employed to regulate buildup, its configuration may be of any suitable shape such as a cylinder or belt. The effectiveness of the brush in controlling metal salt buildup depends upon factors such as the relative speed between the surface of the reusable photoreceptor and the outer periphery of the brush, the diameter of the brush fibers, the length of the brush fibers, the number of brush fibers per square inch, and the amount of brush interference with the surface of the reusable photoreceptor. For example, satisfactory regulation of metal salt buildup on the surface of a reusable photoreceptor is maintained by employing a cylindrical brush having a diameter of about 4 inches, a 10 to about 20 denier polypropylene fiber height of about one-fourth to about onehalf inches and a fiber density of 45,000 to about 60,000 fibers per square inch, rotated at about 2 to about 350 revolutions per minute while maintaining a fiber photoreceptor surface interference at about 0.1 inches. Any suitable durable fibrous material may be employed in the brush. Typical fiber materials include: nylon, polypropylene, Dynel, Arnel and the like. The direction of fibrous wiping member movement may be the same as or opposite to the direction of the photoreceptor surface.

Reusable imaging surface wear may further be reduced by intemiittently bringing the wiping member into contact with the imaging surface. Where the wiping blade embodiment is employed, a reduction in overall toner consumption can be realized by lifting the blade away from the reusable imaging surface at predetermined intervals to allow developer material which may have collected against the wiping blade to be carried by the imaging surface into the developing zone where it is returned to the developer mixture.

The metal salt deposit regulating technique of the instant invention may be employed with any imaging surface including any suitable photoconductive surface. Well known photoconductive materials include vitreous selenium, selenium alloys, organic or inorganic photoconductors embedded in a nonphotoconductive matrix, organic or inorganic photoconductors embedded in a photoconductive matrix, or the like. Representative patents in which photoconductive materials are disclosed include U.S. Pat. No. 2,803,542 to Ullrich, U.S. Pat. No. 2,970,906 to Bixby, U.S. Pat. No. 3,121,066 to Middleton, U.S. Pat. No. 3,121,007 to Middleton, and U.S. Pat. No. 3,151,982 to Corrsin. Reusable photoreceptor surfaces comprising vitreous selenium, selenium alloys or selenium mixtures are preferred because their faster photographic response allows higher machine speeds to be attained.

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples further define, describe and compare the methods of this invention for regulating the metal salt thickness on reusable electrostatic imaging surfaces. Parts and percentages are by weight unless otherwise indicated.

In the following, examples 1 through V1 are carried out in a Xerox 914 copying machine in which the cleaning brush mechanism is replaced with a unit which permits the brush speed and brush to drum distance to be varied.

EXAMPLE 1 About 0.05 parts of zinc stearate having a particle size distribution from about 0.75 microns to about 40 microns is gently folded into one part of Xerox 914 toner particles having an average particle size of about 10 to about 15 microns. The resulting treated mixture is then thoroughly milled in a Szegvari attritor for about 10 minutes. A Xerox 914 copying machine is then charged with about 1 part of the treated toner mixture and about 99 parts Xerox 914 carrier beads having an average particle size of about 500 microns. The cleaning station in the copying machine comprises a standard 3'1-inchdiameter Xerox 914 rabbit fur cleaning brush which is positioned to permit a 0.1-inch fiber interface with the drum surface and operated at 1,200 revolutions per minute. The xerographic drum is rotated at the normal operating speed of about 2 inches per second. After about 1,000 copies are made, the copies and xerographic drum surface are examined for quality and wear, respectively. The copies made near the termination of the test are characterized by low density images and high background toner deposits. The surface of the xerographic drum is covered with a deposit of zinc stearate having a thickness greater than about microns. ln addition, the soft standard rabbit fur cleaning brush employed in the Xerox 914 copying machine is matted and clogged with zinc stearate.

EXAMPLE 11 The developing procedure of example I is repeated under substantially the same conditions except that the standard rabbit fur cleaning brush is replaced with a brush having an overall diameter of about 4 inches, a IS-denier polypropylene pile height of about three-eighth inches, and a fiber density of about 54,000 fibers per square inch. The brush is positioned adjacent a fresh xerographic drum to allow a fiber interference of about 0.1 inches and is rotated at about 175 revolutions per minute. The developer employed in the copying machine is substantially identical to the developer used in example l. After about 80,000 copies are made, the copies and xerographic drum surface are examined for quality and wear, respectively. The copies fon'ned near the termination of the test are characterized by high-density images and substantially no background toner deposits. The drum shows very slight signs of wear and is found to be coated with a zinc stearate film having a thickness of about 3 microns.

EXAMPLE Ill The developing procedure of example I is repeated with a fresh substantially identical brush, new xerographic drum and untreated Xerox 914 toner and carrier. After about 25,000 copies are prepared, the xerographic drum is examined. The selenium surface of the drum shows slight signs of wear.

EXAMPLE V1 The developing procedure of example 11 is repeated with a fresh substantially identical brush, new xerographic drum and untreated Xerox 914- toner and carrier. After about 10,000 copies are made, the xerographic drum is examined. The drum shows signs of severe wear. EXAMPLE V The developing procedure of example 11 is repeated under substantially the same conditions except that the l5-denier brush is replaced with a brush having an overall diameter of about 4 inches, at 7-denier polypropylene pile height of about three-eighth inches, and a fiber density of about 26,000 fibers per square inch. The brush is positioned adjacent a fresh xerographic drum to allow a fiber interference of about 0.07 inches and is rotated at about 1,100 revolutions per minute. After about 75,000 copies are made, the copies and xerographic drum surface are examined for quality and wear, respectively. The copies formed near the termination of the test are characterized by dark dense images and minimal background toner deposits. The drum appears to be only slightly worn and is found to be coated with a zinc stearate iilm having a thickness of about 2 microns.

EXAMPLE VI The developing procedure of example V is repeated under substantially the same conditions except that the polypropylene brush is replaced with a brush having an overall diameter of about 4 inches, a 7 denier nylon pile height of about seven-sixteenths inches, and a fiber density of about 30,000 fibers per square inch. The brush is positioned adjacent a fresh xerographic drum to allow a fiber interference of about 0.1 1 inches and is rotated at about 1,100 revolutions per minute. After about 75,000 copies are prepared, the co pies and xerographic drum surface are examined for quality and wear, respectively. The copies made near the termination of the test are characterized by dense images and almost complete freedom from background deposits. The drum is only slightly worn and is found to be coated with a zinc stcarate film having a thickness of about 3 microns.

EXAMPLE vn About 0.01 parts of calcium palmitate having a particle size distribution from about 1 to about 30 microns is milled for 15 minutes with about one part of pigmented styrene copolymer toner particles described in example 1 of US. Pat. No. 3,079,342. About 1 part of the resulting toner mixture mixed with 99 parts of coated sand carrier beads having an average diameter of about 400 microns is employed to develop electrostatic latent images on a cylindrical photoreceptor surface having a surface speed of about 6 inches per second. A palmitate thickness regulating brush having an overall diameter of about 4 inches, a 15-denier polypropylene pile height of about five-sixteenth inches and a fiber dlensity of about 56,000 fibers per square inch is employed. The brush is positioned adjacent the photoreceptor surface to allow a fiber interference of about 0.09 inches and is rotated at about 300 revolutions per minute. Each developed image is transferred by electrostatic means to a sheet of paper whereon it is fused by heat. After the copying process is repeated about 75,000 times, the copies and photoreceptor surface are examined for quality and wear, respectively. The copies made near the termination of the test contain sharp dense images and a very slight amount of background deposits. The drum shows little sign of wear and is found to be coated with a calcium palmitate film having a thickness of about 3 microns.

EXAMPLE V111 A control sample containing one part colored preformed styrene copolymer toner particles having an average particle size of about 10-12 microns and 99 parts carrier having an average particle size of about 250 microns is cascaded across an electrostatic image-bearing drum surface. The developed image is then transferred by electrostatic means to a sheet of paper whereon it is fused by heat. The residual powder is removed from the electrostatic imaging; surface by a cleaning web of the type disclosed by W. P. Graff, Jr. et al. in US. Pat. No. 3,186,838. A nonwoven rayon'web contact pressure of about 18 pounds per square inch, web-photoreceptor relative speed of about 1.5 inches per second, and a web contact are distance of about one-eighth inch are employed. After the copying process is repeated 5,000 times, the copies and electrostatic image-bearing surface are examined for quality and wear, respectively. The copies possess sharp line contrast and minimal background deposition. However, large solid areas possess a washed out appearance. Micrograph studies of the electrostatic image-bearing surface reveal a great many deep scratches.

EXAMPLE IX About 002 parts of zinc stearate having a particle size distribution from about 0.75 microns to about 40 microns is gently folded into one part of a colored preformed styrene copolymer described in example Vlll. The resulting developer mixture is then thoroughly milled in a Szegvari attritor for about 10 minutes. The developing procedure of example Vll is repeated with a new drum and with the foregoing milled mixture substituted for the toner of example V111. Copies prepared with the milled sample possess higher density solid area coverage and cleaner background than the copies prepared with the control sample. Further, micrograph studies of the electrostatic image-bearing surface indicate shallower and fewer scratches by a factor greater than two in com parison to the scratched image-bearing surface of example Vlll. The drum is found to be coated with a zinc stearate film having a thickness of about 3 microns.

EXAMPLE X About one part zinc stearate having a particle size distribution from about 0.75 microns to about 40 microns is gently folded into one part of a colored. performed styrene copolymer described in example Vlll. The resulting mixture is then rolled in a sealed container for minutes. About one part of the rolled mixture is mixed with 99 parts of a carrier having an average particle size of about 250 microns. The resulting developer mixture is employed in a cascade developing process as described in example Vlll. A film of zinc stearate having a thickness greater than about 25 microns builds up on the surface of the electrostatic image-bearing drum surface and renders the web cleaner substantially ineffective. Copies made with this developing composition contain extremely low density images and excessive quantities of background deposits.

EXAMPLE XI The procedure described in example VIII is repeated except that the drum speed is doubled and the web contact pressure increased to about 28 pounds per square inch. The relative speed between the web and photoreceptor surface is about 3 inches per second. The original drive motor is retained to drive the drum at twice the original speed; the increase in speed being effected by an alteration in the drive ratio. An attempt to operate the altered system is abandoned when the drive motor begins the altered system is abandoned when the drive motor begins to overheat.

EXAMPLE XII The procedure described in example XI is repeated except that 0.03 parts of zinc stearate having a particle size distribution from about 0.075 microns to about 40 microns is added to the colored preformed styrene copolymer toner and carrier mixture. After one hundred copying cycles no evidence of drive motor overheating is observed.

EXAMPLE XII] The procedure described in example VIII is repeated except that the cleaning web is replaced by a rectangular one-sixteenth inch thick strip of square edged vulcanized rubber. The rubber strip is positioned parallel to the axis of the drum with the angular comer formed by a face and long edge of the rectangular strip riding on the surface of the drum. The vertical resultant force employed to press the entire blade against the drum surface is about three pounds as read on a spring scale. After the copying process is repeated about 700 times, the drum imaging surface is examined for wear. The selenium surface of the drum is found to be extensively eroded. Streaks of toner material are observed on the drum surface and the copy images formed indicate insufficient cleaning of the drum surface.

EXAMPLE XIV The procedure described in example XIII is repeated except that about 0.04 parts zinc stearate having a particle size distribution from about 0.75 microns to about 40 microns is added to the colored preformed styrene copolymer toner and carrier mixture. After 700 copying cycles, the drum is examined for wear. The drum shows little sign of wear, image quality is very good and the drum is found to be coated with zinc stearate film having a thickness of about 4 microns.

The expression developing material" as employed herein is intended to include electroscopic toner material or combinations of toner material and carrier material.

Although specific materials and conditions were set forth in the above exemplary processes in making and using the developer materials and metal salt film thickness regulators of this invention, these are merely intended as illustrations of the present invention. Various other toners, carriers, regulators,

substituents and processes such as those listed above may be substituted for those in the examples with similar results.

Other modifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What IS claimed rs:

1. An imaging process comprising steps of:

a. fonning an electrostatic latent image on a reusable imaging surface,

b. applying a free-flowing xerographic developing material comprising .particles, said particles including finely divided toner material having an average particle size less than about 30 microns and from about 0.02 percent to about 20 percent based on the weight of said toner material, of at least one solid, stable, substantially water insoluble, hydrophobic metal salt of a fatty acid having a melting point of at least 57 C. available at external surfaces of said particles, to said electrostatic latent image to form a toner image corresponding to said electrostatic latent image,

c. transferring said toner image from said reusable imaging surface to a support surface, and

d. wiping said reusable imaging surface with at least one wiping member with sufficient pressure to maintain a slippery film of said metal salt of a fatty acid on said reusable imaging surface of a thickness below about 10 microns.

2. An imaging process according to claim 1 wherein said metal salt and said finely divided toner material are freely movable in relation to each other immediately prior to contact with said reusable imaging surface.

3. An imaging process according to claim 1 wherein said metal salt is immovably attached to said finely divided toner material.

4. An imaging process according to claim 1 wherein said particles include carrier particles having said finely divided toner particles electrostatically clinging thereto, said carrier particles having a size such that by their own weight they are movable away from said electrostatically clinging finely divided toner material.

5. An imaging process according to claim 1 wherein said metal salt is zinc stearate.

6. An imaging process according to claim 1 wherein said metal salt is cadmium stearate.

7. An imaging process according to claim 1 wherein said wiping member comprises at least one wiper blade.

8. An imaging process according to claim 7 further including the step of periodically lifting said wiping member away from said reusable imaging surface.

9. An imaging process according to claim 1 wherein said wiping member is fibrous.

10. An imaging process according to claim 1 wherein said pressure is sufficient to maintain said thickness of said film below about 6 microns.

11. An imaging process according to claim 1 wherein said pressure is sufficient to maintain said thickness of said film below about 3 microns.

12. An imaging process according to claim 1 wherein steps (a) through (d) are repeated at least once.

13. An imaging process according to claim 1 wherein said developing material comprises from about 0.05 to about 4 percent, based on the weight of said toner material, of at least one solid, stable, substantially water-insoluble, hydrophobic metal salt of a fatty acid available at external surfaces of said particles, said metal salt having a melting point of at least about 57 C. 

2. An imaging process according to claim 1 wherein said metal salt and said finely divided toner material are freely movable in relation to each other immediately prior to contact with said reusable imaging surface.
 3. An imaging process according to claim 1 wherein said metal salt is immovably attached to said finely divided toner material.
 4. An imaging process according to claim 1 wherein said particles include carrier particles having said finely divided toner particles electrostatically clinging thereto, said carrier particles having a size such that by their own weight they are movable away from said electrostatically clinging finely divided toner material.
 5. An imaging process according to claim 1 wherein said metal salt is zinc stearate.
 6. An imaging process according to claim 1 wherein said metal salt is cadmium stearate.
 7. An imaging process according to claim 1 wherein said wiping member comprises at least one wiper blade.
 8. An imaging process according to claim 7 further including the step of periodically lifting said wiping member away from said reusable imaging surface.
 9. An imaging process according to claim 1 wherein said wiping member is fibrous.
 10. An imaging process according to claim 1 wherein said pressure is sufficient to maintain said thickness of said film below about 6 microns.
 11. An imaging process according to claim 1 wherein said pressure is sufficient to maintain said thickness of said film below about 3 microns.
 12. An imaging process according to claim 1 wherein steps (a) through (d) are repeated at least once.
 13. An imaging process according to claim 1 wherein said developing material comprises from about 0.05 to about 4 percent, based on the weight of said toner material, of at least one solid, stable, substantially water-insoluble, hydrophobic metal salt of a fatty acid available at external surfaces of said particles, said metal salt having a melting point of at least about 57* C. 